Power supply control device and precharge processing method

ABSTRACT

Upon receiving a start instruction, a control device turns on first and third system main relays, and performs precharge processing for charging a capacitor. Here, a limiting resistor for preventing an inrush current into the capacitor in a discharged state at the time of start is not provided in a load drive device. The control device controls a gate voltage of a power MOSFET of the third system main relay such that the power MOSFET operates in a saturation region, in a range not exceeding a maximum rated power.

This nonprovisional application is based on Japanese Patent ApplicationNo. 2005-330168 filed with the Japan Patent Office on Nov. 15, 2005, theentire contents of which are hereby incorporated by reference.

1. Field of the Invention

The present invention relates to a power supply control device and aprecharge processing method, and more particularly to a power supplycontrol device and a precharge processing method for performingprecharge processing for preventing generation of an inrush current whenpower supply from a DC power supply to a load device is started.

2. Description of the Background Art

FIG. 12 is an overall block diagram of a load drive device including aconventional power supply control device. Referring to FIG. 12, a loaddrive device 200 includes a DC power supply 210, system main relays 220to 240, a limiting resistor 250 connected in series to system main relay240, an inverter 260 receiving supply of electric power from DC powersupply 210 and driving a motor-generator 270, a capacitor 280 smoothingan input voltage of the inverter, and a control device 290.

System main relay 240 includes a power transistor and a diode connectedthereto in an anti-parallel manner. System main relay 240 and limitingresistor 250 connected in series are connected in parallel to systemmain relay 230.

Here, DC power supply 210, system main relays 220 to 240, limitingresistor 250, and control device 290 form the power supply controldevice in load drive device 200, and inverter 260, motor-generator 270and capacitor 280 form the load device receiving supply of electricpower from the power supply control device in load drive device 200.

When a start instruction of load drive device 200 is given to controldevice 290, control device 290 initially turns on system main relay 220and the power transistor of system main relay 240. Then, a circuit froma positive electrode to a negative electrode of DC power supply 210through system main relay 220, capacitor 280, system main relay 240, andlimiting resistor 250 is formed, and charging from DC power supply 210to capacitor 280 is started.

Here, as limiting resistor 250 is provided in load drive device 200, aninrush current from DC power supply 210 to capacitor 280 is prevented,and welding of system main relays 220, 230 can be prevented.

As charging of capacitor 280 proceeds, control device 290 turns onsystem main relay 230, and thereafter turns off the power transistor ofsystem main relay 240.

Thus, in the power supply control device above, generation of the inrushcurrent is prevented by providing limiting resistor 250, however,limiting resistor 250 that is used only at the time of start of thesystem has been one of factors to increase the cost.

Japanese Patent Laying-Open No. 5-111240 proposes a power supply controldevice capable of preventing an inrush current without providing alimiting resistor. Japanese Patent Laying-Open No. 5-111240 discloses atechnique to prevent the inrush current while suppressing an effectivevalue of a flow-in current into a DC-DC converter by causing atransistor provided between a main battery and the DC-DC converter toperform PWM (Pulse Width Modulation) operation.

Meanwhile, Japanese Patent Laying-Open No. 2003-92807 discloses atechnique to charge a capacitor with a constant current using aprecharge circuit provided between a battery for driving and an inverterand to protect a transistor within the precharge circuit by reducing theconstant current when a temperature of the transistor within theprecharge circuit attains a temperature equal to or higher than acertain level.

The technique disclosed in Japanese Patent Laying-Open No. 5-111240,however, remains as the technique simply to cause the transistor toperform PWM operation in order to prevent the inrush current, andmaximum rating, a temperature of the transistor and the like are notconsidered.

In addition, the precharge circuit disclosed in Japanese PatentLaying-Open No. 2003-92807 has a complicated circuit configuration, andreduction in cost is not sufficient.

SUMMARY OF THE INVENTION

The present invention was made to solve the above-described problems. Anobject of the present invention is to provide a power supply controldevice and a precharge processing method with low cost, withoutincluding a limiting resistor for preventing an inrush current.

Another object of the present invention is to provide a power supplycontrol device and a precharge processing method preventing overheat ofa system main relay, without including a limiting resistor forpreventing an inrush current.

According to the present invention, a power supply control deviceincludes: a DC power supply; a relay provided between one electrode ofthe DC power supply and a load device; a semiconductor switching elementconnected in parallel to the relay; and a control unit performingprecharge processing for supplying electric charges from the DC powersupply to the load device through the semiconductor switching elementbefore turn-on of the relay. The control unit controls a control voltageof the semiconductor switching element during the precharge processingsuch that power loss of the semiconductor switching element does notexceed maximum rated power of the semiconductor switching element.

In the power supply control device of the present invention, theprecharge processing is performed in order to prevent the inrush currentfrom the DC power supply to the load device. The control unit controlsthe control voltage of the semiconductor switching element during theprecharge processing such that the power loss of the semiconductorswitching element does not exceed the maximum rated power of thesemiconductor switching element. Therefore, generation of the inrushcurrent is prevented without providing the limiting resistor forpreventing the inrush current and without adding other circuits.

Therefore, according to the present invention, the power supply controldevice with low cost, without including a limiting resistor forpreventing the inrush current can be achieved. In addition, the powersupply control device capable of preventing overheat of thesemiconductor switching element, without including a limiting resistorcan be achieved.

Preferably, the semiconductor switching element is implemented by afield-effect transistor. The control unit controls a gate voltage of thefield-effect transistor such that the field-effect transistor operatesin a saturation region.

In the power supply control device, the gate voltage of the field-effecttransistor is controlled such that the field-effect transistor operatesin the saturation region. Therefore, the field-effect transistor of asmall capacity can be selected. Thus, according to the presentinvention, the cost can further be reduced.

In addition, preferably, the semiconductor switching element isimplemented by a bipolar transistor. The control unit controls a basevoltage of the bipolar transistor such that the bipolar transistoroperates in an active region.

In the power supply control device, the base voltage of the bipolartransistor is controlled such that the bipolar transistor operates inthe active region. Therefore, the bipolar transistor of a small capacitycan be selected. Thus, according to the present invention, the cost canfurther be reduced.

In addition, according to the present invention, a power supply controldevice includes: a DC power supply; a relay provided between oneelectrode of the DC power supply and a load device; a semiconductorswitching element connected in parallel to the relay; a control unitperforming precharge processing for supplying electric charges from theDC power supply to the load device through the semiconductor switchingelement before turn-on of the relay; and a temperature detection unitdetecting a temperature of the semiconductor switching element. Thecontrol unit controls a control voltage of the semiconductor switchingelement during the precharge processing such that an amount of currentflow in the semiconductor switching element is decreased with theincrease in the temperature of the semiconductor switching element.

In the power supply control device according to the present invention,during the precharge processing, when the temperature of thesemiconductor switching element is raised, an amount of current thatflows in the semiconductor switching element is decreased. Therefore,according to the present invention, the power supply control devicecapable of reliably preventing overheat of the semiconductor switchingelement without including the limiting resistor for preventing theinrush current can be achieved.

In addition, according to the present invention, a power supply controldevice includes: a DC power supply; a relay provided between oneelectrode of the DC power supply and a load device; a semiconductorswitching element connected in parallel to the relay; a control unitperforming precharge processing for supplying charges from the DC powersupply to the load device through the semiconductor switching elementbefore turn-on of the relay; and a temperature detection unit detectinga temperature of the semiconductor switching element. The control unitsubjects the semiconductor switching element to switching control duringthe precharge processing when the temperature of the semiconductorswitching element is raised.

In the power supply control device according to the present invention,during the precharge processing, when the temperature of thesemiconductor switching element is raised, the control unit subjects thesemiconductor switching element to switching control. Accordingly, anaverage amount of current that flows in the semiconductor switchingelement is decreased. Therefore, according to the present invention, thepower supply control device capable of reliably preventing overheat ofthe semiconductor switching element without including the limitingresistor for preventing the inrush current can be achieved.

Preferably, the control unit lowers on-duty of the semiconductorswitching element during the precharge processing with the increase inthe temperature of the semiconductor switching element.

In the power supply control device, during the precharge processing, theaverage amount of current that flows in the semiconductor switchingelement is decreased with the increase in the temperature of thesemiconductor switching element. Therefore, according to the presentinvention, overheat of the semiconductor switching element can reliablybe prevented, while appropriately suppressing an amount of current flowin the semiconductor switching element.

In addition, according to the present invention, a precharge processingmethod relates to precharge processing for performing precharge from apower supply device to a load device. The power supply device includes aDC power supply, a relay provided between one electrode of the DC powersupply and the load device, and a semiconductor switching elementconnected in parallel to the relay. The precharge processing methodincludes the first to fourth steps. In the first step, a control voltageof the semiconductor switching element is calculated such that powerloss of the semiconductor switching element does not exceed maximumrated power of the semiconductor switching element. In the second step,the calculated control voltage is output to a control electrode of thesemiconductor switching element. In the third step, whether prechargeperformed through the semiconductor switching element is completed ornot is determined. In the fourth step, if it is determined thatprecharge is completed, the relay is turned on.

In addition, according to the present invention, a precharge processingmethod relates to precharge processing for performing precharge from apower supply device to a load device. The power supply device includes aDC power supply, a relay provided between one electrode of the DC powersupply and the load device, a semiconductor switching element connectedin parallel to the relay, and a temperature detection unit detecting atemperature of the semiconductor switching element. The prechargeprocessing method includes the first to fifth steps. In the first step,a detected temperature is obtained from the temperature detection unit.In the second step, a control voltage of the semiconductor switchingelement is calculated such that an amount of current flow in thesemiconductor switching element is decreased with the increase in theobtained detected temperature. In the third step, the calculated controlvoltage is output to a control electrode of the semiconductor switchingelement. In the fourth step, whether precharge performed through thesemiconductor switching element is completed or not is determined. Inthe fifth step, if it is determined that precharge is completed, therelay is turned on.

In addition, according to the present invention, a precharge processingmethod relates to precharge processing for performing precharge from apower supply device to a load device. The power supply device includes aDC power supply, a relay provided between one electrode of the DC powersupply and the load device, a semiconductor switching element connectedin parallel to the relay, and a temperature detection unit detecting atemperature of the semiconductor switching element. The prechargeprocessing method includes the first to fourth steps. In the first step,a detected temperature is obtained from the temperature detection unit.In the second step, when the obtained detected temperature is raised,the semiconductor switching element is subjected to switching control.In the third step, whether precharge performed through the semiconductorswitching element is completed or not is determined. In the fourth step,if it is determined that precharge is completed, the relay is turned on.

As described above, according to the present invention, the power supplycontrol device with low cost, without including a limiting resistor forpreventing the inrush current can be achieved. In addition, the powersupply control device capable of preventing overheat of a system mainrelay, without including a limiting resistor for preventing the inrushcurrent can be achieved.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overall block diagram of a load drive device including apower supply control device according to the first embodiment of thepresent invention.

FIG. 2 is an equivalent circuit diagram of the power supply controldevice during precharge processing.

FIG. 3 is a characteristic diagram of a power MOSFET shown in FIG. 1.

FIG. 4 is a flowchart of the precharge processing performed by thecontrol device shown in FIG. 1.

FIG. 5 is an overall block diagram of a load drive device including apower supply control device according to a variation of the firstembodiment of the present invention.

FIG. 6 is a characteristic diagram of a bipolar transistor shown in FIG.5.

FIG. 7 is an overall block diagram of a load drive device including apower supply control device according to the second embodiment of thepresent invention.

FIG. 8 shows a gate voltage of a power MOSFET controlled by the controldevice shown in FIG. 7.

FIG. 9 is a flowchart of precharge processing performed by the controldevice shown in FIG. 7 FIG. 10 shows on-duty of a power MOSFETcontrolled by a control device in the third embodiment.

FIG. 11 is a flowchart of precharge processing performed by the controldevice in the third embodiment.

FIG. 12 is an overall block diagram of a load drive device including aconventional power supply control device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention will be described hereinafter indetail with reference to the drawings. The same or correspondingelements in the drawings have the same reference characters allotted,and therefore detailed description thereof will not be repeated.

First Embodiment

FIG. 1 is an overall block diagram of a load drive device including apower supply control device according to the first embodiment of thepresent invention. Referring to FIG. 1, a load drive device 100 includesa DC power supply B, system main relays SMR1 to SMR3, a capacitor C, aninverter 10, a control device 20, a power supply line PL, and a groundline SL.

System main relay SMR1 is connected between a positive electrode of DCpower supply B and power supply line PL. System main relay SMR2 isconnected between a negative electrode of DC power supply B and groundline SL. System main relay SMR3 includes a power MOSFET (metal oxidesemiconductor field-effect transistor) 40 and a diode D connected inparallel to power MOSFET 40. System main relay SMR3 is connected inparallel to system main relay SMR2, between the negative electrode of DCpower supply B and ground line SL.

Capacitor C is connected between power supply line PL and ground lineSL. Inverter 10 includes a U-phase arm, a V-phase arm and a W-phase arm(not shown) connected in parallel between power supply line PL andground line SL. Connection points between upper arms and lower arms ofU-, V- and W-phase arms are connected to U-, V- and W-phase coils (notshown) of a motor-generator MG driven by load drive device 100,respectively.

DC power supply B, system main relays SMR1 to SMR3 and control device 20form the power supply control device in load drive device 100, andinverter 10, motor-generator MG and capacitor C form the load devicethat receives supply of power from the power supply control device inload drive device 100.

DC power supply B is implemented, for example, by a secondary batterysuch as a nickel metal hydride battery or a lithium-ion battery, andsupplies DC power to power supply line PL through the system main relay.It is noted that DC power supply B may be implemented by a capacitor ofa large capacity or a fuel cell.

System main relays SMR1, SMR2 are implemented by a mechanical relay or asemiconductor relay. On the other hand, system main relay SMR3 isimplemented by the semiconductor relay (power MOSFET 40) as describedabove. System main relays SMR1 to SMR3 are controlled by signals SE1 toSE3 from control device 20 respectively. Specifically, system mainrelays SMR1, SMR2 are turned on by signals SE1, SE2 at H (logic high)level respectively, and turned off by signals SE1, SE2 at L (logic low)level respectively. In addition, power MOSFET 40 of system main relaySMR3 receives signal SE3 at the gate terminal, and varies the draincurrent in accordance with the voltage level of signal SE3.

Capacitor C smoothes voltage fluctuation between power supply line PLand ground line SL. Inverter 10 converts a DC voltage received frompower supply line PL to an AC voltage based on a signal PWMI fromcontrol device 20, and outputs the resultant voltage to motor-generatorMG.

Upon receiving a start signal ST instructing start of load drive device100, control device 20 performs the precharge processing for chargingcapacitor C in the discharged state. Specifically, upon receiving startsignal ST, control device 20 initially turns on system main relays SMR1,SMR3, and as charging of capacitor C proceeds, control device 20 turnson system main relay SMR2.

Here, a limiting resistor for preventing an inrush current intocapacitor C in the discharged state at the time of start of the systemis not provided in load drive device 100. In order to prevent the inrushcurrent, control device 20 controls a gate voltage of power MOSFET 40 ofsystem main relay SMR3 such that power MOSFET 40 operates in asaturation region, in a range not exceeding the maximum rated power.

In addition, when the precharge processing ends, control device 20starts control for driving motor-generator MG. Specifically, controldevice 20 generates signal PWMI for driving motor-generator MG based ona torque instruction, a motor current of each phase and an inverterinput voltage of motor-generator MG, and outputs generated signal PWMIto inverter 10. The motor current of each phase and the inverter inputvoltage of motor-generator MG are detected by a current sensor and avoltage sensor respectively, -both of which are not-shown.

FIG. 2 is an equivalent circuit diagram of the power supply controldevice during the precharge processing. Referring to FIG. 2, during theprecharge processing, a closed circuit as shown is formed. Here, aresistor RB shows the internal resistance of DC power supply B.

Assuming that a voltage across terminals of DC power supply B is denotedas E, a value of the internal resistance is denoted as r, a drainvoltage of power MOSFET 40 is denoted as VDS, and a drain current isdenoted as ID, drain current ID of power MOSFET 40 at the time of startof the precharge processing (that is, the voltage across terminals ofcapacitor C being set to 0V) is expressed as in Equation (1) below.Drain current ID=(E−VDS)/r  (1)

FIG. 3 is a characteristic diagram of power MOSFET 40 shown in FIG. 1.Referring to FIG. 3, the abscissa represents drain voltage VDS of powerMOSFET 40, and the ordinate represents drain current ID. Regions shownwith solid lines k1, k2 and k3, that is, regions where drain current IDhardly depends on drain voltage VDS and exhibits the constant currentcharacteristic determined only by gate voltage VGS, are generallyreferred to as a “saturation region”, and a region shown with a solidline k4, that is, the region where drain current ID significantlyincreases with drain voltage VDS, is generally referred to as a“non-saturation region”.

A dotted line k5 shows the maximum rated power of power MOSFET 40. Adotted line k6 shows relation between drain voltage VDS and draincurrent ID at the time of start of the precharge processing, andcorresponds to Equation (1) above. A dotted line k7 shows, ascomparison, relation between drain voltage VDS and drain current ID whena limiting resistor (resistance value R) for inrush current preventionis connected in series to the power MOSFET.

In the conventional power supply control device including the limitingresistor, resistance value R of the limiting resistor is determined suchthat dotted line k7 is lower than dotted line k5 showing the maximumrated power of the power MOSFET. Then, gate voltage VGS of the powerMOSFET is set to VGS1 such that drain current ID at the time of start ofthe precharge processing is set to a value corresponding to a point P1(maximum value).

In contrast, in the case of the power supply control device in the firstembodiment without including the limiting resistor, if gate voltage VGSof the power MOSFET is set to VGS2 such that drain current ID at thetime of start of the precharge processing is set to a valuecorresponding to a point P2 (maximum value), power loss of power MOSFET40 exceeds the maximum rated power of power MOSFET 40 shown with dottedline k5, and power MOSFET 40 may break.

Then, in the first embodiment, gate voltage VGS of power MOSFET 40 isset to VGS3 such that drain current ID at the time of start of theprecharge processing is set to a value corresponding to a point P3.Thus, power loss of power MOSFET 40 during the precharge processing canbe suppressed to a value not higher than the maximum rated power shownwith dotted line k5, and power MOSFET 40 can be prevented from breaking.

In addition, by setting gate voltage VGS of power MOSFET 40 to VGS3,power MOSFET 40 operates in the saturation region. Therefore, in thefirst embodiment, the power MOSFET of a capacity smaller than in theconventional power supply control device where the power MOSFET operatesin the non-saturation region (on solid line k4) can be selected, and asmaller size and lower cost of the device can be achieved.

FIG. 4 is a flowchart of the precharge processing performed by controldevice 20 shown in FIG. 1. Referring to FIG. 4, control device 20determines whether an instruction to start load drive device 100 hasbeen issued or not based on start signal ST (step S10). If controldevice 20 determines that the start instruction has not been issued (NOin step S10), control device 20 ends a series of processing.

If it is determined in step S10 that the instruction to start load drivedevice 100 has been issued (YES in step S10), control device 20calculates the gate voltage of power MOSFET 40 of system main relay SMR3(step S20). Specifically, control device 20 uses, for example, thecharacteristic diagram of FIG. 3 to calculate gate voltage VGS3corresponding to point P3 at which Equation (1) above is satisfied andthe maximum rated power of power MOSFET 40 is not exceeded.

Thereafter, control device 20 outputs the calculated gate voltage to thegate terminal of power MOSFET 40 of system main relay SMR3, and turns onsystem main relay SMR1 (step S30). Then, charging from DC power supply Bto capacitor C is started through system main relay SMR1 and powerMOSFET 40 of system main relay SMR3.

When charging of capacitor C is completed (YES in step S40), controldevice 20 turns on system main relay SMR2 (step S50). Thereafter,control device 20 sets the gate voltage that has been output to powerMOSFET 40 to 0, turns off system main relay SMR3 (step S60), and ends aseries of precharge processing.

In the description above, during the precharge processing, the gatevoltage of power MOSFET 40 of system main relay SMR3 is fixed to gatevoltage VGS3 calculated in step S20. On the other hand, if it isdetermined in step S40 that charging of capacitor C is not completed (NOin step S40), control device 20 may return again to step S20, and thegate voltage of power MOSFET 40 may successively be operated.

An effect of successive operation of the gate voltage of power MOSFET 40is as follows. Referring again to FIG. 3, as charging of capacitor Cproceeds, relation between drain voltage VDS and drain current ID makestransition in a direction of lower drain voltage VDS from point P3 alongsolid line k3. Therefore, margin for the maximum rated power shown withdotted line k5 becomes greater. Then, by successively operating the gatevoltage of power MOSFET 40, relation between drain voltage VDS and draincurrent ID can make transition along the maximum rated power shown withdotted line k5 (successively calculated gate voltage VGS graduallyincreases). Accordingly, drain current ID of power MOSFET 40 can bemaximized in a range not exceeding the maximum rated power, and the timefor precharge processing can be shortened.

As described above, according to the first embodiment, as the limitingresistor for preventing the inrush current can be dispensed with, thepower supply control device with low cost can be achieved. In addition,as the gate voltage of power MOSFET 40 is controlled such that powerloss of power MOSFET 40 of system main relay SMR3 does not exceed themaximum rated power, overheat of power MOSFET 40 can be prevented.

Moreover, as power MOSFET 40 operates in the saturation region, powerMOSFET 40 of a smaller capacity can be selected, and consequently asmaller size and lower cost of the power supply control device can beachieved.

Variation of the First Embodiment

FIG. 5 is an overall block diagram of a load drive device including apower supply control device according to a variation of the firstembodiment of the present invention. Referring to FIG. 5, a load drivedevice 100A includes a system main relay SMR3A instead of system mainrelay SMR3 in the configuration of load drive device 100 shown inFIG. 1. System main relay SMR3A includes a bipolar transistor 50 insteadof power MOSFET 40 in the configuration of system main relay SMR3 shownin FIG. 1. Bipolar transistor 50 is implemented, for example, by an IGBT(Insulated Gate Bipolar Transistor).

FIG. 6 is a characteristic diagram of bipolar transistor 50 shown inFIG. 5. Referring to FIG. 6, the abscissa represents a collector voltageVCE of bipolar transistor 50, and the ordinate represents a collectorcurrent IC. Regions shown with solid lines k11, k12 and k13, that is,regions where collector current IC hardly depends on collector voltageVCE and exhibits the constant current characteristic determined only bya base voltage VBE, are generally referred to as an “active region”, anda region shown with a solid line k14, that is, the region wherecollector current IC significantly increases with collector voltage VCE,is generally referred to as a “linear region” (or “saturation region”).

A dotted line k15 shows the maximum rated power of bipolar transistor50. A dotted line k16 shows relation between collector voltage VCE andcollector current IC at the time of start of the precharge processing. Adotted line k17 shows, as comparison, relation between collector voltageVCE and collector current IC when the limiting resistor (resistancevalue R) for inrush current prevention is connected in series to thebipolar transistor.

As shown in FIG. 6, the voltage-current characteristic the same as inthe case of power MOSFET 40 shown in FIG. 3 is also exhibited in bipolartransistor 50. Therefore, by setting base voltage VBE of bipolartransistor 50 to VBE3 such that collector current IC at the time ofstart of the precharge processing is set to a value corresponding to apoint P13, power loss of bipolar transistor 50 can be suppressed to avalue not higher than the maximum rated power shown with dotted linek15, and bipolar transistor 50 can be prevented from breaking.

In addition, by setting base voltage VBE of bipolar transistor 50 toVBE3, bipolar transistor 50 operates in the active region. Therefore, inthe variation of the first embodiment as well, the bipolar transistor ofa small capacity can be selected, and a smaller size and lower cost ofthe device can be achieved.

As described above, an effect the same as in the first embodiment can beobtained also according to the variation of the first embodiment.

Second Embodiment

FIG. 7 is an overall block diagram of a load drive device including apower supply control device according to the second embodiment of thepresent invention. Referring to FIG. 7, a load drive device 100B furtherincludes a temperature sensor 30 and includes a control device 20Ainstead of control device 20 in the configuration of load drive device100 in the first embodiment shown in FIG. 1.

Temperature sensor 30 is arranged in the vicinity of power MOSFET 40 ofsystem main relay SMR3, detects a temperature T of power MOSFET 40, andoutputs the temperature to control device 20. For example, temperaturesensor 30 may be implemented by a sensor that detects a temperature ofpower MOSFET 40 using a thermistor, or by a sensor that detects atemperature by utilizing temperature dependency of the voltage acrossterminals of the diode arranged in the proximity of power MOSFET 40.

Control device 20A performs the precharge processing upon receivingstart signal ST, as in the case of control device 20 in the firstembodiment. Here, when the precharge processing is started, controldevice 20A sets gate voltage VGS of power MOSFET 40 of system main relaySMR3 to the maximum voltage. As temperature T of power MOSFET 40detected by temperature sensor 30 is raised, however, control device 20Alowers gate voltage VGS with the increase in the temperature. Thus,drain current ID of power MOSFET 40 is suppressed, and temperatureincrease in power MOSFET 40 is suppressed. Gate voltage VGS of powerMOSFET 40 is supplied to power MOSFET 40 as signal SE3, as in the firstembodiment.

FIG. 8 illustrates a gate voltage of power MOSFET 40 controlled bycontrol device 20A shown in FIG. 7. Referring to FIG. 8, whentemperature T of power MOSFET 40 exceeds a temperature TO indicating thetemperature increase thereof, control device 20A lowers gate voltage VGSof power MOSFET 40 from a maximum voltage V0 with the increase intemperature T. Here, maximum voltage V0 is comparable to gate voltageVGS2 corresponding to point P2, in the characteristic diagram of powerMOSFET 40 shown in FIG. 3.

FIG. 9 is a flowchart of the precharge processing performed by controldevice 20A shown in FIG. 7. Referring to FIG. 9, the processingconfiguration shown in the flowchart includes steps S22, S24 instead ofstep S20 in the processing configuration shown in FIG. 4. Namely, if itis determined in step S10 that the instruction to start load drivedevice 100B has been issued (YES in step S10), control device 20Aobtains temperature T of power MOSFET 40 of system main relay SMR3detected by temperature sensor 30 from temperature sensor 30 (step S22).

Then, control device 20A calculates gate voltage VGS of power MOSFET 40of system main relay SMR3 based on temperature T of power MOSFET 40obtained from temperature sensor 30 (step S24). Specifically, forexample, relation between temperature T of power MOSFET 40 and gatevoltage VGS shown in FIG. 8 is defined in a map in advance, and controldevice 20A uses the map to calculate gate voltage VGS based ontemperature T from temperature sensor 30. Then, control device 20Aproceeds to step S30.

During the precharge processing, control device 20A successivelycalculates gate voltage VGS based on temperature T from temperaturesensor 30. Namely, if it is determined in step S40 that charging ofcapacitor C is not completed (NO in step S40), control device 20Areturns to step S22.

In the description above, power MOSFET 40 is used as the semiconductortransistor for precharge, however, bipolar transistor 50 may be usedinstead of power MOSFET 40 as in the variation of the first embodiment.

As described above, according to the second embodiment, the limitingresistor for preventing the inrush current is not included, and the gatevoltage of power MOSFET 40 is controlled such that an amount of currentthat flows in power MOSFET 40 is decreased when the temperature of powerMOSFET 40 used as the relay for precharge is raised. Therefore, overheatof power MOSFET 40 can reliably be prevented.

Third Embodiment

In the second embodiment, when the temperature of power MOSFET 40 ofsystem main relay SMR3 is raised, gate voltage VGS of power MOSFET 40 islowered with the increase in the temperature thereof In the thirdembodiment, when the temperature of power MOSFET 40 is raised, powerMOSFET 40 is turned on/off, and on-duty of power MOSFET 40 is loweredwith the temperature increase in power MOSFET 40.

The entire configuration of the load drive device in the thirdembodiment is the same as that of load drive device 100B in the secondembodiment shown in FIG. 7.

FIG. 10 illustrates on-duty D_ON of power MOSFET 40 controlled bycontrol device 20B in the third embodiment. Referring to FIG. 10, whentemperature T of power MOSFET 40 exceeds temperature TO indicatingtemperature increase thereof, control device 20B subjects power MOSFET40 to switching control such that an average amount of current thatflows in power MOSFET 40 is decreased. Here, control device 20B lowerson-duty D_ON of power MOSFET 40 with the temperature increase in powerMOSFET 40.

FIG. 11 is a flowchart of the precharge processing performed by controldevice 20B in the third embodiment. Referring to FIG. 11, the processingconfiguration shown in the flowchart includes steps S26, S32 instead ofsteps S24, S30 respectively in the processing configuration shown inFIG. 9. Namely, when temperature T of power MOSFET 40 of system mainrelay SMR3 is obtained from temperature sensor 30 in step S22, controldevice 20B calculates on-duty D_ON of power MOSFET 40 of system mainrelay SMR3 based on the temperature of power MOSFET 40 obtained fromtemperature sensor 30 (step S26). Specifically, for example, relationbetween temperature T of power MOSFET 40 and on-duty D_ON shown in FIG.10 is defined in a map in advance, and control device 20B uses the mapto calculate on-duty D_ON based on temperature T from temperature sensor30.

When on-duty D_ON of power MOSFET 40 is calculated, control device 20Bsubjects power MOSFET 40 to switching control at calculated on-dutyD_ON, and turns on system main relay SMR1 (step S32). Then, controldevice 20B proceeds to step S40.

During the precharge processing, control device 20B successivelycalculates on-duty D_ON of power MOSFET 40 based on temperature T fromtemperature sensor 30. Namely, if it is determined in step S40 thatcharging of capacitor C is not completed (NO in step S40), controldevice 20B returns to step S22.

In the description above as well, power MOSFET 40 is used as thesemiconductor transistor for precharge, however, bipolar transistor 50may be used instead of power MOSFET 40 as in the variation of the firstembodiment.

As described above, according to the third embodiment, the limitingresistor for preventing the inrush current is not included, and powerMOSFET 40 is subjected to switching control such that an average amountof current that flows in power MOSFET 40 is decreased when thetemperature of power MOSFET 40 used as the relay for precharge israised. Therefore, overheat of power MOSFET 40 can reliably beprevented.

In each embodiment above, system main relay SMR3 for prechargeprocessing is assumed to be connected in parallel to system main relaySMR2 connected to the negative electrode of DC power supply B, however,it may be connected in parallel to system main relay SMR1 connected tothe positive electrode of DC power supply B.

In the description above, system main relay SMR1 or SMR2 corresponds tothe “relay” in the present invention, and power MOSFET 40 of system mainrelay SMR3 or bipolar transistor 50 of system main relay SMR3Acorresponds to the “semiconductor switching element” in the presentinvention. In addition, capacitor C, inverter 10 and motor-generator MGform the “load device” in the present invention, and control devices 20,20A and 20B correspond to the “control unit” in the present invention.Moreover, DC power supply B, system main relays SMR1 to SMR3 (or SMR3A)and control device 20 (or 20A, 20B) form the “power supply controldevice” in the present invention. Further, temperature sensor 30corresponds to the “temperature detection unit” in the presentinvention.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

1. A power supply control device comprising: a DC power supply; a relayprovided between one electrode of said DC power supply and a loaddevice; a semiconductor switching element connected in parallel to saidrelay; and a control unit performing precharge processing for supplyingelectric charges from said DC power supply to said load device throughsaid semiconductor switching element before turn-on of said relay; andsaid control unit being configured to control a control voltage of saidsemiconductor switching element during said precharge processing suchthat power loss of said semiconductor switching element does not exceedmaximum rated power of said semiconductor switching element.
 2. Thepower supply control device according to claim 1, wherein saidsemiconductor switching element is implemented by a field-effecttransistor, and said control unit controls a gate voltage of saidfield-effect transistor such that said field-effect transistor operatesin a saturation region.
 3. The power supply control device according toclaim 1, wherein said semiconductor switching element is implemented bya bipolar transistor, and said control unit controls a base voltage ofsaid bipolar transistor such that said bipolar transistor operates in anactive region.
 4. A power supply control device comprising: a DC powersupply; a relay provided between one electrode of said DC power supplyand a load device; a semiconductor switching element connected inparallel to said relay; a control unit performing precharge processingfor supplying electric charges from said DC power supply to said loaddevice through said semiconductor switching element before turn-on ofsaid relay; and a temperature detection unit detecting a temperature ofsaid semiconductor switching element; and said control unit beingconfigured to decrease an amount of current flow in said semiconductorswitching element as the temperature of said semiconductor switchingelement is higher during said precharge processing by controlling acontrol voltage of said semiconductor switching element.
 5. A powersupply control device comprising: a DC power supply; a relay providedbetween one electrode of said DC power supply and a load device; asemiconductor switching element connected in parallel to said relay; acontrol unit performing precharge processing for supplying charges fromsaid DC power supply to said load device through said semiconductorswitching element before turn-on of said relay; and a temperaturedetection unit detecting a temperature of said semiconductor switchingelement; and said control unit being configured to subject saidsemiconductor switching element to switching control during saidprecharge processing when the temperature of said semiconductorswitching element is raised.
 6. The power supply control deviceaccording to claim 5, wherein said control unit lowers on-duty of saidsemiconductor switching element during said precharge processing, withincrease in the temperature of said semiconductor switching element. 7.A precharge processing method of performing precharge from a powersupply device to a load device, said power supply device including a DCpower supply, a relay provided between one electrode of said DC powersupply and said load device, and a semiconductor switching elementconnected in parallel to said relay, comprising: the first step ofcalculating a control voltage of said semiconductor switching elementsuch that power loss of said semiconductor switching element does notexceed maximum rated power of said semiconductor switching element; thesecond step of outputting the calculated control voltage to a controlelectrode of said semiconductor switching element; the third step ofdetermining whether said precharge performed through said semiconductorswitching element is completed; and the fourth step of turning on saidrelay when it is determined that said precharge is completed.
 8. Theprecharge processing method according to claim 7, wherein saidsemiconductor switching element is implemented by a field-effecttransistor, and in said first step, a gate voltage of said field-effecttransistor is calculated such that said field-effect transistor operatesin a saturation region.
 9. The precharge processing method according toclaim 7, wherein said semiconductor switching element is implemented bya bipolar transistor, and in said first step, a base voltage of saidbipolar transistor is calculated such that said bipolar transistoroperates in an active region.
 10. A precharge processing method ofperforming precharge from a power supply device to a load device, saidpower supply device including a DC power supply, a relay providedbetween one electrode of said DC power supply and said load device, asemiconductor switching element connected in parallel to said relay, anda temperature detection unit detecting a temperature of saidsemiconductor switching element, comprising: the first step of obtaininga detected temperature from said temperature detection unit; the secondstep of calculating a control voltage of said semiconductor switchingelement such that an amount of current flow in said semiconductorswitching element is decreased with increase in the obtained detectedtemperature; the third step of outputting the calculated control voltageto a control electrode of said semiconductor switching element; thefourth step of determining whether said precharge performed through saidsemiconductor switching element is completed; and the fifth step ofturning on said relay when it is determined that said precharge iscompleted.
 11. A precharge processing method of performing prechargefrom a power supply device to a load device, said power supply deviceincluding a DC power supply, a relay provided between one electrode ofsaid DC power supply and said load device, a semiconductor switchingelement connected in parallel to said relay, and a temperature detectionunit detecting a temperature of said semiconductor switching element,comprising: the first step of obtaining a detected temperature from saidtemperature detection unit; the second step of subjecting saidsemiconductor switching element to switching control when the obtaineddetected temperature is raised; the third step of determining whethersaid precharge performed through said semiconductor switching element iscompleted; and the fourth step of turning on said relay when it isdetermined that said precharge is completed.
 12. The prechargeprocessing method according to claim 11, wherein in said second step,said semiconductor switching element is subjected to switching controlsuch that on-duty of said semiconductor switching element is loweredwith increase in said detected temperature.